1. Field
This application relates generally to a method and device for reducing noise generated by supersonic jet exhaust.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A supersonic jet exhaust nozzle generally includes an upstream converging section and a downstream diverging section. The converging section expands and accelerates subsonic exhaust gas flow such that the exhaust gas flow reaches the speed of sound at a location of minimum cross-sectional area known as the geometric throat of the nozzle. The diverging section of the nozzle, which begins downstream from the throat, further expands and accelerates the exhaust gas flow to supersonic speeds.
A difference in pressure between an ambient air mass and supersonic exhaust gases departing the nozzle at a nozzle exit, causes cone-shaped waves of compression and expansion, or “Mach cones,” to form in an exhaust plume formed downstream from the nozzle exit. The Mach cones are formed as the plume undergoes pressure equalization with the ambient air. Mach cones created in the jet exhaust are propagated downstream along the jet exhaust plume. The Mach cones include shock waves, which generate a significant amount of noise as they impinge upon the shear layer between the exhaust plume and the ambient air.
The amount of noise generated by a jet engine's exhaust plume may, therefore, vary according to (among other factors) an expansion state of an engine's exhaust nozzle, which governs the exhaust plume's pressure relative to that of the ambient air mass. A jet engine exhaust nozzle may have an ideally expanded, under-expanded, or over-expanded state. An under-expanded nozzle is a nozzle shaped such that jet engine exhaust gas pressure at a nozzle exit plane is higher than ambient atmospheric pressure. In contrast, an over-expanded nozzle is a nozzle shaped such that jet engine exhaust gas pressure at the nozzle exit plane is lower than ambient atmospheric pressure. A perfectly or ideally expanded nozzle is a nozzle shaped in such a way that jet engine exhaust gas pressure at the nozzle exit plane equals ambient atmospheric pressure, thereby precluding the formation of Mach cones.